XPost: comp.mobile.android, alt.os.linux, alt.comp.os.windows-10
From: dan@djph.net
Pat wrote:
> On Fri, 14 Apr 2017 16:35:16 -0000 (UTC), Tomos Davies
> wrote:
>
>>I agree with all of you that it's more elegant to assigned a fixed IP at
>>the DHCP configuration of the access point.
>>
>>One open question I have is what happens under the current circumstances
>>when the Android device requests an IP address of 192.168.1.15 of the
>>access point ... if ... if the access point has already handed out that IP
>>address to another device?
>
> I'm jumping in here without having read the whole thread, but I want
> to comment on your "open" question:
>
> Devices do NOT request a certain IP address. [...]
Actually, they can and do. The caveat is that they MUST[1] have already
received an IP address from somewhere. In addition, I believe the
address also has to have some lease time remaining (although, this may
not be a hard requirement).
For example, you take your phone to work every day. When you get home,
and the phone re-connects to your home's wifi, the DHCP DISCOVER packet
will (typically) include the last known IP address as part of the
request -- essentially "I had 10.11.12.13 before, can I have it again?"
The DHCP OFFER (server response to the client's discover) will either
confirm the client can use its requested IP address, or tell it to use
something else (e.g. 192.168.1.144).
To wrap up, the client sends the DHCP REQUEST and server sends DHCP ACK
packets.
Same thing happens if you reboot your computer (though with a higher
chance of the DHCP server allowing re-use of the requested IP address).
[1] as per RFC2119 -- https://tools.ietf.org/rfc/rfc2119.txt
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